And now we have a head's-up for every cellphone owner. Over the last year, one out of every three robberies has involved the theft of a phone. Tonight, police are asking phone companies to take some action. Abc's linsey davis explains. Reporter: This woman, knocked over, punched, then slammed against the wall -- all for her cell phone. This woman's cell phone -- snatched right out of her hands. It happened to tatiana santos just on tuesday. The guy just snatched the phone out of my hand. And kept running. Reporter: When tatiana tried to use an app to locate her stolen phone, it didn't work. Hing they did, of course, was turn off the phone, so there was no way of locating the phone anymore. Reporter: Just by powering it off, thieves can prevent your phone from being tracked, and with basic technology they can wipe data from the phone and resell it for as much as a thousand dollars. It's just that easy. The wireless phone industry is in the process of putting a central database online to track stolen phones and prevent them from being used. But police we spoke to in san francisco, where nearly half of all robberies involved a cellphone, they told us carriers should do more. They want the phone companies to render a kill switch, which would render the phone useless in the u.S. San francisco police say the phone companies aren't aggressively doing everything they can to solve the problem because they get more money when your phone is stolen. them to watch tv, surf the internet, or chat with someone face to face -- if only they had the technology to dissuade a thief. Linsey davis, abc news, new york.

This transcript has been automatically generated and may not be 100% accurate.

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